1. Technical Field of the Invention
The invention relates to elevator active suspensions and, more particularly, to control of magnetic actuators.
2. Discussion of Related Art
It is known from U.S. Pat. No. 5,439,075, for example, to control horizontal motions of an elevator car guided vertically along hoistway rails by means of an active suspension system. The guiding means can be provided in the form of roller clusters at the corners of the car for engaging the hoistway rails on opposite walls of the hoistway. Horizontal acceleration of the elevator car and horizontal displacement between the car and the rail is sensed for controlling the horizontal motions by means of actuators of the active suspension system. Each roller cluster may include one or more actuators with associated springs wherein the roller cluster actuators are responsive to a controller for actuating the elevator car horizontally with respect to the associated hoistway rail.
A controller shown in FIG. 20 of the above mentioned U.S. patent includes a summer responsive to a force command signal and to a force feedback signal for providing a force error signal to a proportional-plus-integral gain compensator. The compensator in turn provides a current command signal to a current driver which provides current to a coil of an electromagnet actuator of the active suspension. This current in the coil is sensed by a sensor and provided along with a sensed magnetic flux signal to a signal processor for providing a signal indicative of the size of an airgap between the electromagnet and an iron reaction plate. Another signal processor, i.e., a flux-to-force converter, is responsive to the sensed magnetic flux signal for providing the force feedback signal (which is simply related to the square of the flux) to the summer.
As can be seen at column 17, lines 63-66 and the proportional gain of the compensator 486 of FIG. 20 of the above-mentioned U.S. patent, is a constant. Unfortunately, the output force characteristic of an electromagnet actuator is a doubly non-linear function of current and gap. Consequently, the open loop gain of such a force loop varies tremendously over the operational ranges of current and gap and can cause instabilities at the extremes. The performance of the force loop is thereby limited to worst-case gain considerations.